As is known, unicast is a point-to-point distribution, whereby one stream of data exists per recipient on every link connecting a given recipient and the source.
Multicasting is a service that permits a source to send a message to a plurality of specific recipients. The notion multicasting ideally involves that only one copy of the message will pass over any link in a network and copies of the message will be made only where paths diverge. From a network perspective, multicasting dramatically reduces overall bandwidth consumption, since data is replicated in the network at appropriate points.
Receivers join a particular multicast session group by informing a corresponding multicast router whereby traffic is delivered to all members of that group by the network infrastructure. The transmission can be performed either from one user to many users, the so-called one-to-many multicast or many users send information to many receivers, the so-called many-to-many transmission.
For typical local area networks, LAN, utilising CSMA (Carrier Sense Multiplex Access) where nodes share a common communication medium, multicasting is easy to support. A specially addressed packet can be read off the communication medium by multiple hosts. Ethernet, for instance, supports uni-cast addresses, multicasting addresses and the network broadcast address.
Extending multicasting capabilities to inter-networks however led to the introduction of a router at the edge of a network in order to figure out dynamically how to forward the datagram, whereby a datagram denotes a data packet, which is established on the IP layer. The way of forwarding is deducted from the address included in the header of the datagram and from the routing table, which is administrated by the router.
With Internet Protocol IP multicast, receivers do not need to know who or where the senders are and senders do not need to know who the receivers are. Neither senders nor receivers need to care about the network topology as the network optimises delivery. The distribution of information via IP multicast is performed on the base of a hierarchical connection of the hosts, like for example a tree. Several algorithms have been proposed for building multicast distribution trees, like for example spanning trees, shared-trees, source-based trees, core-based trees. The descriptions of the corresponding algorithms can be found in “IP telephony: Packet-based multimedia communications systems” O. Hersent, D. Gurle, D. Petit, Addison-Wesley, Harlow, 2000. After the establishment of the distribution tree, the distribution of the information is done by the IP multicast routing protocols.
A specific IP multicast address denotes that a given datagram belongs to a multicast group rather than the addresses of individual recipients, as is the case with normal IP addresses. There are various possibilities of assigning multicast addresses to multicast groups. The task of a given multicast router is to encode the destination addresses and to route multicast datagrams according to information in its routing table.
For IP multicast, the membership of a multicast session group may be dynamic, that is, hosts may join and leave groups at any time. Hence, multicast routers need to update information on which other hosts and routers are present members of a given multicast group.
The Internet Group Management Protocol, IGMP, is a protocol that allows inter-domain routing of IP multicast data. Multicast routers use IGMP to learn which multicast groups have members on each of their attached physical networks. IGMP allows group termination to be quickly reported to the routing protocol, which is important for high-bandwidth multicast groups and sub-nets with highly volatile group membership.
According to IGMP, multicast routers periodically send a General Query to the all systems multicast group (224.0.0.1) (address) in order to determine if other multicast routers are present on the segment. A mechanism secures that only one multicast router adopts the state of Querying multicast router on a given segment. The querying router issues Group Specific Queries. In response, hosts send a Membership Report, informing whether the host is a member of the particular multicast group to the Querying router. When a router on the other hand receives a Report, it adds the group being reported to the list of multicast group memberships on the network on which it received the Report and sets a timer for membership. When a host joins a multicast group, it immediately transmits an unsolicited membership report. IGMP has been specified in Network Working Group RFC 2236.
Multicast Listener Discovery, MLD, is an alternative protocol to IGMP that allows the use of IPv6(Internet Protocol version 6). In MLD, a Multicast Listener Report message corresponds to the Membership Report of IGMP. Furthermore according to MLD, the message Multicast Listener Done, corresponds to a Membership Report. MLD is specified in Network Working Group RFC 2710.
Currently there is activity within the 3'rd generation partnership project, 3GPP, regarding the introduction and support of multicast services in packet data mobile systems.
According to known GPRS and UMTS networks, the mobile terminated down-link traffic is routed using identities which are designed to only handle point to point traffic.
As is known in the art, in order to perform unicast transport, a data structure denoted mobility management (MM) context exists in the mobile station and the SGSN. Moreover, a Packet data protocol (PDP), context, which e.g. comprises an IP address for allowing the mobile station to participate in an IP session with an Internet service provider exist at the mobile station, the SGSN and the GGSN.
The technical background about GPRS in GSM and UMTS is covered in the following:    3GPP TS 03.60V7.5.0(2001-01) 3rd Generation Partnership Project;    Technical Specification Group Services and System Aspects,    Digital cellular Telecommunications System (Phase 2+),    General Packet Radio Service (GPRS),    Service Description, Stage 2(Release 1998).    3GPP TS 23.060V3.6.0(2001-01)    3rd Generation Partnership Project;    Technical Specification Group Services and System Aspects,    General Packet Radio Service (GPRS),    Service Description, Stage 2(Release 1999).